The present invention relates generally to apparatus for injecting alloying ingredients into a stream of molten metal and more particularly to apparatus for injecting an alloying ingredient having a relatively low melting point into a stream of molten steel.
Among the alloying ingredients added to steel to improve its machinability are low melting point ingredients such as lead or bismuth. In the continuous casting of molten steel, a stream of molten steel flows downwardly from a ladle into a tundish. Lead in the form of shot may be added to this stream. It is desirable to enclose the entirety of the stream within a shroud and add the lead to the stream inside that shroud.
Such an arrangement is disclosed in Rellis, et al., U.S. Pat. No. 4,602,949 issued July 29, 1986. In that arrangement there is an inner conduit and a concentric outer shroud. The inner conduit or tube extends downwardly from the bottom of the ladle and a stream of molten steel flows through this inner conduit. The outer shroud is concentric with and radially spaced from the inner conduit and extends from above the bottom of the inner conduit to below the top surface of the bath of molten steel in the tundish. The bottom of the inner conduit terminates substantially above the bottom of the outer shroud, and the lead shot is injected into the stream of molten steel, below the bottom of the inner conduit, through a feed nozzle which extends angularly downwardly through the wall of the outer shroud.
The feed nozzle is composed of a metal such as stainless steel. The upstream end of the feed nozzle is connected to a line or hose through which the lead shot is conducted by a transport gas. By injecting lead shot into the stream of molten steel within the confines of the outer shroud, the escape of lead fumes into the surrounding atmosphere is minimized. Additional information on this apparatus and the method in which it is employed is disclosed in said Rellis, et al. patent, and the disclosure thereof is incorporated herein by reference.
The shroud is composed of refractory material, and the temperature within the shroud interior is relatively high. This causes the nozzle to heat up, and there is a decreasing temperature gradient extending upstream from the nozzle outlet at the downstream end of the nozzle. This can cause premature melting of the lead shot, in the nozzle, and can also cause lead shot, at locations upstream of the nozzle outlet to become sticky or tacky. As a consequence, there can be a build-up of lead within the nozzle, at a location upstream of the nozzle outlet, eventually causing a lead flow blockage within the nozzle.
There is another problem which can arise when employing a concentric conduit and shroud arrangement of the type described above. More particularly, the inner conduit has an upper open end communicating with an opening in a rotary gate mounted on the bottom of the ladle. Periodically, this gate is rotated between open and closed positions, e.g. at the beginning and the end of a cast, and at times in between. When the rotary gate is in its open position, molten metal will flow out of the ladle, through an opening in the gate and through the inner conduit of the double shroud. When the rotary gate is in a closed position, no molten metal will flow out of the ladle.
When the top of the inner conduit is connected to the rotary gate, and the rotary gate is rotated, the inner conduit and everything connected thereto, including the outer shroud and the nozzle extending through the outer shroud, will rotate about the axis of the rotary gate. Any flexible lines or hoses connected to the nozzle (e.g. the lead shot feed line) will be moved as the nozzle rotates about the axis of the rotary gate. As a result of such movement, a flexible line connected to the nozzle can become bent, twisted or kinked, and this would interfere with flow through that line. In addition, there are opposing pulls or forces acting on the nozzle, due to the urging of the shroud, on the one hand, and the drag of the flexible lines attached to the nozzle, on the other hand. These opposing forces can create stresses on the shroud at the location where the nozzle extends through the shroud. This can cause cracking or breaking or be otherwise injurious to the shroud at that location.